1. Field of Invention
The present invention relates generally to the field of broadcast transmitters which will be converted from analog amplitude modulation (AM) to digital modulation as digitalization moves forward, and particularly to a method for avoiding spurious emissions in AM transmitters for digital transmission.
In this context, the intention is for the hitherto usual transmitter types, non-linear AM transmitters featuring an RF input (radio frequency) and an audio input, to continue in use. The reasons for this are as follows:                AM transmitters internally operate in switched mode and therefore have efficiencies which are better by a factor of 3 than those of linear transmitters which are otherwise usually used for digital transmission, for example, in the case of DAB (Digital Audio Broadcasting) and DVB (Digital Video Broadcasting). This results in a saving of operating costs.        it is easier to convince broadcasters to convert from analog to digital if no great investments come up in the preliminary stages.        
2. Description of the Related Art
The digitalization of AM broadcasting is seen as the only chance to preserve these frequency ranges and the technology used therein in the long term. For implementation purposes, the consortium “Digital Radio Mondiale” was founded, see “Rundfunktechnische Mitteilungen” [Broadcasting Newsletter], 43rd year, 1999, issue 1, pages 29-35.
The use of a non-linear AM transmitter for digital modulation requires a special operating mode of the transmitter. The modulated digital signal is generated by two partial signals (I and Q), which are orthogonal to each other. The I-signal (“in phase”) is modulated onto a cosine oscillation having the frequency Ft (carrier frequency). The Q-signal (“quadrature”) is modulated onto a sine oscillation having the same frequency Ft. The sum of both modulated oscillations produces the complex modulated data signal (cosine 0.180 degrees, sine 90−+90 degrees). The modulated I/Q-signal is shaped by filters in such a manner that it has exactly the prescribed curve shape with the desired bandwidth.
For non-linear operation, it is required for the modulated I/Q-signal to be converted in such a manner that the two signals amplitude signal (A-signal) and phase-modulated carrier signal (RF-P) result therefrom which are suitable for proper control of the AM transmitter. Then, at the output of the AM transmitter, the modulated I/Q-signal is generated again with higher power.
The modulated I/Q-signal corresponds to a Cartesian representation. The Cartesian representation is converted to a polar representation with amplitude and phase. In this manner, the amplitude signal (A-signal) is obtained to control the AM transmitter at the audio input. A phase-modulated radio frequency (RF-P signal) is generated from the initially resulting phase signal (P-signal). Advantageously, the RF-P signal can also be directly obtained without the intermediate step via the P-signal. In this manner, the signals are obtained that are required for controlling the AM transmitter:                amplitude signal (A-signal)        phase-modulated RF signal (RF-P signal)        
The A-signal is fed into the modulator input (audio input) of the AM transmitter, and the RF-P signal is used for HF-type control of the transmitter. In the transmitter output stage, the two signals A & RF-P are multiplicatively combined, forming the high frequency digital output signal.
Due to the required conditioning process, both the A-signal and the RF-P signal obtain far larger bandwidths than the one the digital signal originally had and is intended to have again at the output of the transmitter.
Older modulators are frequently not able to provide the increased bandwidths (3-5 times the bandwidth in the AM mode of the transmitter) because they were not designed for this. When using only the limited bandwidth that “older” transmitters have available in the modulator section, then this results in considerable out-of-band and spurious emissions. These have the property that they have only a very small gradient in the spectrum and therefore interfere with quite a number of adjacent channels.
Thus, the band limitation in the amplitude branch (A-branch) and also in the RF-branch results in spurious emissions whose shoulder distance in relation to the spectrum of the data channel does not or only very inadequately meet the requirements of the ITU.